F Tremolada & J. R. Young: Volume calculation of nannofossils ... , p. 199 - 202.

Journal of Nannoplankton Research, 24, 3, 2002.

VOLUME CALCULATION OF CRETACEOUS CALCAREOUS
NANNOFOSSILS
Fabrizio Tremolada* & Jeremy R. Youngf*Dipt. di Scienze della Terra, Univ. di Milano, Via Mangiagalli 34, 1-20133 Milano, ltalx,fabrizio. tremol ada@unim i. it;
fPalaeontology Dept., The Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Abstract: Volume estimates are calculated for common mid-Cretaceous nannofossils, using a methodology
previously developed from studies of extant nannoplankton. The range of volume variation within thisgroup is highlighted, and the potential of such estimates for giving new insights into palaeoecological andpalaeoceanographic studies is outlined.

Introduction

Nannofossils vary in linear size by about an order ofmagnitude, consequently they vary in volume by about
three orders of magnitude, i.e. 1000-fold. In traditionalpalaeoecological analyses, this variation is ignored,
however, there is obviously potential for investigating
whether volume-calibrated census-data may yield moremeaningful data, especially for studies of carbonateformation.
Several estimates of the volume of extant
coccoliths have been published, primarily to allow
conversion of coccolith fluxes into carbonate fluxes(Paasche, 1962; Honjo, 1976; Samtleben & Bickert, 1990;
Faggerbakke et al., 1994; Beaufort & Heussner, 1999; Young& Ziveri, 2000). These estimates are being widely used in
sediment-trap studies (e.g. Broerse, 2000; Sprengel, 2000).
The purpose of this paper is to produce estimates of thevolume and mass of the most abundant calcareous
nannofossils of the mid-Cretaceous (Barremian-Albian).
From these estimates, the amount of CaC0 3 produced bycoccoliths during interpreted eutrophic and oligotrophic
conditions will, subsequently, be estimated (Tremolada &Erba, in prep.). It is hoped that this will provide a new
approach to the study of a key time-interval which wasaffected by some of the strongest perturbations of Earth's
history: in particular, the widespread deposition of blackshales, and the associated Oceanic Anoxic Events, arecharacteristic of this age.
There have previously been only a few estimatesof the volume of Mesozoic nannofossils. Williams &Bralower (1995) proposed an 'averaged Cretaceous nannofossil' with a volume of 14flm3 • In the study ofMattioli &Pittet (2002), analyses were mostly performed on theJurassic nannolith, Schizosphaerella punctulata. The
present work represents the first attempt to investigate
the significance of volume variation within Cretaceousnannofloras.
Material and methods

The best-preserved samples available were investigatedin order to determine the average dimensions of the mostabundant mid-Cretaceous nannofossils. Samples fromODP Legs 185 (Marianne-Bonin Trench) and 171 (BlakeNose), and DSDP Leg 62 (Mid-Pacific Mountains) werestudied, along with some outcrop samples from the Tethyan
area. The interpreted oligotrophic assemblages are

dominated by Watznaueria barnesiae, Nannoconus spp.,
Zeugrhabdotus embergeri and Rhagodiscus asper,whereas interpreted eutrophic assemblages arecharacterised by high abundances of Zeugrhabdotus
erectus, Discorhabdus rotatorius, Biscutum constans andZeugrhabdotus spp. (interpreted after, e.g., Erba, 1992; Erba
et al., 1992; Erba, 1994).
As noted by Young & Ziveri (2000), the volume
function for any given shape is of the form V=K, * 13 , whereK, is a constant depending on the shape, and I is acharacteristic dimension. Thus, there are essentially twosteps to determining nannofossil volume, frrst size analysis,
to determine average size, second shape analysis, todetermine K, Since volume is proportional to length cubed,
accurate size determination is critical.
For size determination, images of coccoliths and nannoliths
were collected using a digital image-capture system (Younget al., 1996) at 1600x and 1250x magnification, then thedimensions were measured using the software NIH-Image,adapted for nannoplankton analyses. For each taxon, atleast 250 specimens were measured to provide accurate
size estimates.
For shape analysis, two approaches were used.
For coccoliths and nannoconids, the NIH-Image macro
program, described by Young & Ziveri (2000), was used to
calculate volumes of rotation from cross-sections. Given a
digitised cross-section and an axial ratio as inputs, theprogram measures and displays thickness as a function of
distance from the axis of symmetry, calculates the implied
volume, and displays the result. To produce cross-sections,mobile mounts were used in order to tilt individualspecimens; images of these were captured. Outlines of thecross-sections were then traced in a graphics program andexported to NIH-Image for quantification.
For geometric-shaped nannoliths (pentaliths,Assipetra infracretacea, etc.), volumes were calculatedusing appropriate mathematical formulae for average-sizedspecimens. K, values were then calculated. These K, values
can be used to calculate volumes for populations ofdifferent average size.
Results

The results of the volume calculations are presented inTable 1. The following notes give details of the basis forthe calculations.
·199

Journal of Nannoplankton Research , 24, 3 , 2002.

F 7l·emolada & J. R. Young: Volume calculation of nannofossils ... , p. 199 - 202.

volume(microns)

mass

major dimension(microns)

dimensionused

Ks
adoptedKs -range(pi()

COCCOLITHSWatVUlueria barnesiae
Zeugrhabdotus embergeri
Rhagodiscus asperZ. diplogrammus
Z. elegans
Z. erectusDiscorhabdus rotatorius
BiscUIUm constans

2187
14
9,3
5,40,8
4,0
3,2

57
234
39
25
15
2
11
9

6,1
9,0
5,23
5,01
4,43
2,64
3,08
3,43

lengthlengthlengthlengthlengthlengthlengthlength

0,09
0, 12
0,10
0,07
0,06
0,04
0,1 4
0,08

0.067-0.098
0.104-0. 127
0.093-0.1 06
0.052-0.068
0.036-0 .049
0.024-0 .038
0.124-0 . 144
0.063-0.089

NANNOLITHS
Nannoconus steinmannii
N. truittii
N. bucheri

1246
341
664

3364
920
1794

18,40
8,80
11,56

heightheightheight

0,20
0,50
0,43

0.10-0.34
0.37-0.58
0.33-0.52

Rucirwlithus terebrodentarius (smal~· terebrodentarius (large)
f.issipetra infracretacea (small)
A. infracretacea (large)
Micrantholithus hoschulzii
M. obtusus

24,3
270
35,4
323
38
157

66
729
96
872
103
424

4,01
9,76
4,61
9,90
3,00
5,70

width
width
width
width
side lengthside length

0,38
0,29
0,36
0,33
1,41
0,85

speciesgeometric solid thickness
cylinder1.93
cylinder3.61
parallelepiped
2.12
parallelepiped
4.41
pentagonal prism
1.4pentagonal prism
1.9

minor dimension

3,61
7,41
3,60
5.81

Table 1: Volume calculations, also showing K, values

Nannoconids: volumes of the nannoconid groupwere calculated using three approaches: (1) the volumesof rotation approach was applied to several captured crosssections of each species; (2) the volume of rotation wasalso applied to the cross-section drawing ofPerch-Nielsen(1985: see Figure 1); and (3) a simple geometrical estimationof the volume of the total nannoli.th and central canal wasalso performed. Nannoconus steinmannii steinmannii isshaped like the frustum of a cone. The major and minor
diameters, and the height, were measured and the total
volume was calculated. In this species, the canal is cylindershaped and the diameter is about 1J,lm. Nannoconus bucheri
and Nannoconus truittii are cylinder-shaped nannoconids.
They belong to the wide-canal group, and the thickness of
the wall was measured on both cross- and axial-sections.
The results from these different calculations arecompared in Table 2. There is significant range in the shapeconstants for each species but the results of the different
methods are comparable, hence we have calculated a meanof the estimates for each species.

number ofspecies

N. steinmannii

N. truittii'N. bucheri

our sections
PN85 sections
simple volume
average all estimates
our sections
PN85 sections
simple volume
average all estimates
our sections
PN85 sections
simple volume
average all estimatesestimates
182
1
64
152

For coccoliths, the volume of rotation approach
was used for all species. Cross-sections (Figure 1) were
produced using, primarily, captured images from mobile
mounts, and the K, constant was calculated from theseusing the volume of rotation macro of Young & Ziveri(2000). The major axis (length) of elliptical coccoliths wasadopted as the characteristic dimension.
Watznaueria barnesiae displays a wide sizerange. From these analyses, a mean major-axis value of
6.lJ.!m and an ellipticity value (major/minor-axis ratio) of
1.14 was detennined . The calculated volume of 21J.!m 3
should not, however, be used in other studies. Instead,the mean size in the population should be detennined, andan appropriate volume calculated using the K, value. Thisis good practice for all species, but is essential for species
which vary greatly in size.
The central process of Zeugrhabdotus embergeri
was measured, and its volume geometrically calculated.
Unfortunately, no specimens of Rhagodiscus asper showeda well-preserved central process, so we increased thecoccolith volume by 15%.
In the genus Zeugrhabdotus, the bridge
represents c.20% of the total volume. The volume of thecalcified skeleton was calculated as follows :min max mean

VCaC03 =V tot- V area + V bridge

0,1 0,49 0,210 ,3 0,27 0,270,120,210,3 0,8 0,500,4 0,68 0,510,470,500,3 0,52 0,470,4 0,47 0,440,350,42Table 2 : Volume estimates for the Nannoconus species studied,comparing results of different approaches

where V area represents the volume of the inner areadelimited by the rim, and V bridge the volume of the bridge.
The parallelepiped volume formula was used to determine
the bridge volume, where possible. The calculated volumeof Zeugrhabdotus erectus, one of the smallest common
Cretaceous coccoliths, is 0. 76J.!m 3 • This result is inagreement with 0. 9J.!m3, the estimated volume of a ' typicalsmall coccolith' of the Jiving nannoplankton (Young &Ziveri, 2000).
For Assipetra infracretacea (normal- and large-200

F Tremolada & J R. Young: Volume calculation of nannofossils .. ., p. 199 - 202.

N truittiifrequen

PL COLITH

Journal of Nannoplankton Research, 24, 3, 2002.

N teinmannii

N·truittii truittii

PE T

cros - ections

W. borne iae

N bucheri

ITH (oblique view

M. ho ·chulzii

D. rotatorius

M. obtu us
B. con tan
Figure 1: Top - representative nannoconid cross-sections of the studied species, derived from SEM illustrations from Perch-Nielsen(1985). Other cross-sections were traced from photomicrographs. Plots above the profiles show program output: thickness of specimen
through a sequence of increments from the centre of the specimen, and volume of each increment. The program sums the volumeincrements to produce a total volume estimate. Note that the left and right profiles are separately analysed to give two estimates ofvolume. Bottom - reconstructions of placolith cross-sections and oblique views of the most common pentaliths of the mid-Cretaceous

sized morphotypes), a subrectangular-shaped nannolith,
the geometric function for calculating the volume of aparallelepiped was used. Rucinolithus terebrodentarius(normal- and large-sized morphotypes) is composed of adouble whorl of eight imbricate elements. Its shape can beapproximated as cylindrical.

Discussion
Young & Ziveri (2000) estimated the errors in volume
determination as c. 50% . The error percentage forCretaceous nannofloras is inevitably higher due todiagenesis. A comparison of SEMs of even well-preserved
Cretaceous nannofossils with those of extant

nannoplankton suggests that volume increase is pervasive
during diagenesis, and for nannoconids there is the realpossibility that the original nannoliths may have beenhighly porous rather than solid. Consequently, results
should be interpreted with caution, and only broad-scalepatterns interpreted, however, given the very large
variations in volume between species (from <I J..tm 3 to> 1OOOJ..tm 3), even estimates with such high uncertainty canprovide a valuable perspective for palaeoecological andbiogeochemical interpretations of raw count data.
The results (Table 1) quantify a previously-observed
contrast between interpreted oligotrophic and eutrophicindicators. The oligotrophic indicator-species (according201

Journal of N annoplankton Research , 24, 3, 2002.

F Tremolada & J. R. Young: Volume calculation of nannofossil s ... , p. 199 - 202.

to previous interpretations) have the largest volumes andmasses, whereas the higher fertility indicators (according
to previous interpretations) are smaller, and so have much
lower volumes. Based on such estimates of coccolith mass,
it may be possible to investigate biogenic fluxes,
characterising lower- and higher-fertility conditions,recorded in different lithologies (Tremolada & Erba, inprep.). In addition, these estimates may be used to help
investigate the response ofphytoplankton to nutrification
events and to calibrate the real contribution ofnannoplankton to pelagic carbonates.

Acknowledgements
The authors thank Jackie Lees and Paul Bown for theirreviews of the manuscript.

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